1. Field of the Invention
This invention relates to a connection assembly between an electrically insulative tubular member and an electrically conductive end cap, typically in the context of an electrical device such as an oil-submersible current limiting fuse (CLF) or a lightning arrestor. More particularly, it is concerned with such an end cap connection assembly making use of a relatively rigid synthetic resin sealing material between the tubular member and metallic end cap which has a characteristic heat distortion temperature analogous to that of the tubular member. Moreover, the synthetic resin sealing material advantageously exhibits a coefficient of expansion profile similar to that of the tubular housing. In this fashion, the end cap connection assembly provides an effective seal throughout all normal thermal cycling of the elctrical device.
2. Description of the Prior Art
During construction of electrical devices, the securement of insulative tubular members to conductive end pieces or caps has long been a challenge to those in the field. Typically, the end caps are comrised of a metallic material such as copper, aluminum or brass for conducting a current of electricity between an external circuit and an electrical subassembly within the device. On the other hand, the insulative member may be a synthetic resin tube having glass fibers, or alternatively may be comprised of a material such as porcelain or the like.
By way of illustration, current limiting fuses as well as lightning arrestors often include an outer, cylindrical, fiberglass tube with a copper end cap assembly secured to each end of the tube. The end cap assembly commonly has either a lug, a male threaded portion or a threaded bore for releasably fixing an electrical lead to the end cap in order to provide a current path to an electrical subassembly disposed within the insulative tube.
As can be appreciated, it is desirable to provide a means for securely connecting the end cap assembly to the insulative tube to prevent relative movement between the end cap and the insulative tube. During installation, the worker may apply substantial torsional stresses to the assembly as the wire lead is coupled to the end cap in an attempt to preclude subsequent unintentional loosening of the electrical joint. Unfortunately, rotative or axial movement of the cap relative to the tube can irreversibly damage portions of the internal electrical subassembly.
Moreover, such electrical devices are often placed in service in environments which include exposure to fluids. The aforementioned current limiting fuses and lightning arrestors are sometimes immersed in insulating transformer oil within a subgrade or grade level compartment. Alternatively, fuses and lightning arrestors may be mounted atop a utility pole and exposed to rainwater, ice and snow. Consequently, the end cap assembly must sealingly engage the insulating tube in such a manner to enable the device to completely resist the infiltration of fluids over the service life of the device. Moreover, such a seal must not be broken when the device is subject to severe installation stresses.
In the past, a variety of methods have been proposed for securely and sealingly connecting an end cap to an insulative tube. For example, in U.S. Pat. No. 4,063,208 to Bernatt, dated Dec. 13, 1977, an end portion of an insulating tube is provided with a first recess for receiving an O-ring, and a somewhat cup-shaped metallic end piece is mounted over the tube end portion to engage the O-ring. Portions of the end cap are magneformed inwardly to engage the tube in the vicinity of a second tube recess. However, expensive tooling is required to form such recesses, and the tube end portions are weakened somewhat unless the thickness of the tube is increased to provide additional strength. Moreover, the O-ring functions only as a seal and does not contribute to the overall strength of the connection.
U.S. Pat. No. 4,146,862 to Mikulecky, dated Mar. 27, 1979, discloses a connection between an end cap assembly and an insulating tube wherein a gasket having a rounded outermost lip is placed partially within the tube and the latter is inserted into an end cap until the rounded lip is positioned within an annular, epoxy containing groove formed in the end cap. Unfortunately, such construction requires the use of a gasket, thereby raising the costs of the parts and the amount of labor needed for assembly.
Additionally, when the insulating tube is comprised of glass fibers, it is important to prevent fluid from entering the device along leak paths parallel to the disposition of the fibers. During manufacturing of the tube, some of the fibers are sometimes not completely sealed and when the tube is cut to length, breakage of the fiber strands creates additional openings for possible subsequent fluid entry. Accordingly, it is preferable that the ends of fiberglass tubes are completely sealed so that the usable life of the electrical device is not unnecessarily shortened.